President Donald Trump urged the new leader of Ukraine to investigate the son of former Vice President Joe Biden.

The revelation is likely to raise more questions in the ongoing controversy over a mysterious complaint submitted by an intelligence whistleblower that involves Trump's communications with a foreign leader. The complaint has created a showdown between Congress and the White House.

Two people familiar with the matter say the complaint was based on a series of events, one of which was a July 25 call between Trump and Ukrainian president Volodymyr Zelenskiy.

One of the people says Trump urged Zelenskiy to probe the activities of potential rival Biden's son Hunter, who worked for a Ukrainian gas company. The people were not authorized to discuss the issue by name.

Trump is angrily labeling the allegation as "partisan" even as Democrats move to investigate the interactions.

The Wall Street Journal first reported Trump pressured Ukraine's president during a July phone call to investigate Democratic presidential candidate Joe Biden's son Hunter, repeatedly suggesting that he work with Rudy Giuliani, his personal lawyer, to carry out the probe.

The Washington Post also reported that the President pressed the leader of Ukraine to investigate Biden's son during the call, which took place one day after former special counsel Robert Mueller testified before Congress about Russian interference in US elections.

The complaint deals at least in part with Ukraine, The New York Times and Washington Post reported Thursday night.

In the July 25 phone call, Trump told Ukrainian President Volodymyr Zelensky "that he should work with [Mr. Giuliani] on Biden, and that people in Washington wanted to know" whether allegations were true or not, one of the people said, according to the Journal.

Trump suggested Zelensky enlist the help of Giuliani about eight times, the Journal reports. The Post did not report that detail.

The person familiar with the call told the Journal that Trump did not mentioned the issue of foreign aid to Ukraine during the conversation and did not believe the President explicitly offered any quid-pro-quo for Ukraine's cooperation on an investigation.

A readout of the call put out by the Ukrainian government, which occurred one day after Special Counsel Robert Mueller testified before Congress about foreign interference in US elections, references discussion of investigating corruption, but the White House version makes no mention -- saying only that he was congratulating him on his recent election.

Trump downplays whistleblower's claims

Before the Journal's report was published, Trump downplayed the significance of the whistleblower complaint at the center of the latest controversy consuming Washington. During remarks in the Oval Office Friday, Trump claimed the whistleblower is partisan and his conversations with foreign leaders are "appropriate."

But in a back-and-forth with reporters, Trump did not deny discussing Biden with Zelensky in that July phone call -- a potential subject for the complaint.

Now, the Journal is reporting that Trump not only discussed Biden with Zelensky during that phone call but urged the Ukrainian President to use his personal lawyer to help investigate the former vice president's son.

"It's a partisan whistleblower. They shouldn't even have information. I've had conversations with many leaders. They're always appropriate," Trump said prior to the Journal's report being published while appearing alongside the Australian Prime Minister, who he was hosting for a state visit.

Precise details of the accusation aren't known, much to the chagrin of Democratic lawmakers, who have demanded more information.

"To ask a foreign leader to interfere in a presidential election - right after a year long investigation into potential foreign interference in a presidential election - would be pretty extraordinary. Even for these times," Connecticut Democrat Sen. Chris Murphy tweeted Friday in reaction to the news.

Trump admitted Friday he did not know the whistleblower's identity, and said he wasn't even sure if it was his conversation with Ukraine's leader that was the topic of the complaint.

But he nevertheless appeared defensive about his interactions with Zelensky, with whom he will meet next week on the sidelines of the United Nations General Assembly.

Trump still pushing Biden claim

Asked whether he'd discussed Biden with Ukraine's Zelensky during a phone call in late July, Trump said it "doesn't matter what I discussed," but insisted that someone should look into Biden.

"It was disgraceful, where he talked about billions of dollars he's not giving to a certain country unless a certain prosecutor's taken off the case," Trump said, appearing to refer to January remarks when Biden discussed his interactions with Ukraine's leadership when serving as vice president.

Following a Friday town hall in Cedar Rapids, Iowa, Biden said Trump's assertion was unfounded.

"Not one single credible outlet has given any credibility to his assertion. Not one single one, and so I have no comment except the President should start to be president," he said.

Appearing in a rambling interview on CNN on Thursday evening, Giuliani admitted he had discussed Biden with Ukraine's leaders during a visit to the country.

Giuliani has long lobbied Ukraine to investigate Biden's call in 2016 to remove the country's top prosecutor, who at one point had been investigating a Ukrainian natural gas company connected to Biden's son, Hunter.

Other Western governments also called for that prosecutor's dismissal, and no evidence has indicated Biden's move was inappropriate.

Ukraine's prosecutor general told Bloomberg in May he had no proof of wrongdoing by Biden or his son.

Trump's conversation with the Ukrainian President has come under scrutiny from House investigators and they have demanded more information about Trump and Giuliani's interactions regarding Ukraine and the Biden matter, including calls for a transcript of Trump's July phone call with Ukraine's president.

Democrats are also looking for more information about the whistleblower's complaint.

"There's a real sense of urgency here since the Inspector General was so clear this is not something that can wait," said House Intelligence Committee chairman Adam Schiff on Friday, a day after his panel was briefed behind closed doors by the inspector general.